Heart and Vessel Formation: A Detailed Look at Embryonic Development
Cardiovascular System Development
Cardiac Development
The cardiovascular system originates from the mesoderm. Around day 22 of development, a pair of heart tube structures forms, eventually fusing into a single heart tube composed of an inner endocardial tube and a surrounding myocardial layer. Between weeks 4 and 7, the heart develops into its characteristic four-chamber structure.
Formation of the Cardiac Septum
The heart’s septa develop from endocardial cushions in the atrioventricular canal and the troncoconal region.
Atrial Septum
The septum primum descends from the atrial ceiling, creating a space called the ostium primum for communication between the atria. The ostium primum closes as the septum primum fuses with the endocardial cushions, forming the ostium secundum. The secondary septum then develops, leaving the foramen ovale as an interatrial opening.
Atrioventricular Canal Septation
Four endocardial cushions surround the atrioventricular canal. The fusion of the upper and lower cushions divides the opening into the right and left atrioventricular canals. The cushion tissue becomes fibrous, forming the mitral (left) and tricuspid (right) valves.
Ventricular Septation
The interventricular septum consists of a thick muscular portion and a thin membranous portion. The membranous portion comprises a lower atrioventricular endocardial cushion and the left and right edges of the cone.
Bulb Septation
The bulb divides into the trunk (aorta and pulmonary trunk), the cone (infundibulum of the aorta and pulmonary trunk), and the trabeculated portion of the right ventricle. The aortic-pulmonary septum divides the trunk region into the two main arteries. Swelling of the cone separates the aortic and pulmonary outflow channels and closes the interventricular foramen with endocardial cushion tissue.
Vascular Development
Arterial System
Three major derivatives of the original system are:
- The aortic arch (4th aortic arch)
- The pulmonary artery (6th aortic arch), which communicates with the aorta via the ductus arteriosus in utero
- The right subclavian artery, originating from the 4th right aortic arch, distal right dorsal aorta, and the 7th intersegmental artery
The vitelline arteries, initially distributed in the yolk sac, form the celiac trunk and superior and inferior mesenteric arteries, supplying the anterior, middle, and posterior bowel.
The umbilical arteries arise from the iliac arteries. Postnatally, their distal portions obliterate, forming the median umbilical ligaments, while the proximal parts become the internal iliac arteries and supply the bladder.
Venous System
Three systems are recognized:
- The vitelline system, which becomes the portal system
- The cardinal system, forming the vena cava system
- The umbilical system, which disappears after birth
Lymphatic System
The lymphatic system originates from five sacs: two jugular, two iliac, one retroperitoneal, and the cisterna chyli. Numerous channels connect these sacs and drain to other structures. The thoracic duct forms from the anastomosis of the left and right thoracic ducts, the distal right thoracic duct, and the cranial portion of the left thoracic duct. The right lymphatic duct forms from the cranial portion of the right thoracic duct.
Postnatal Changes
- Obliteration of the umbilical arteries
- Obliteration of the umbilical vein (ligamentum teres) and ductus venosus (venous ligament)
- Obliteration of the ductus arteriosus, leaving the ligamentum arteriosum
- Closure of the foramen ovale, caused by increased left atrial pressure pushing the septum primum against the septum secundum, occurring with the newborn’s first cry